1. Field of the Invention
The present invention relates to a food temperature sensing probe assembly for use in a microwave oven. More particularly, the invention relates to such a probe assembly which prevents overcooking of the food in a region immediately adjacent the probe.
2. Description of the Prior Art
Electric thermometer probes which monitor the internal temperature of the food while it is being cooked have been developed for use in microwave ovens. Examples of such probes, adapted to use in microwave ovens, are disclosed and claimed in copending application Ser. No. 616,049, filed Sept. 23, 1975 by David Y. Chen and Louis H. Fitzmayer, now U.S. Pat. No. 3,975,720, issued Aug. 17, 1976; and in copending application Ser. No. 616,050, filed Sept. 23, 1975 by Louis H. Fitzmayer, now U.S. Pat. No. 3,974,696, issued Aug. 17, 1976; each entitled "Food Thermometer for Microwave Oven," and assigned to the same assignee as the present invention. Such probes comprise a needle-like probe adapted to be driven into the food and a temperature-sensing element, such as a thermistor, positioned internally of the probe housing near the distal end thereof. A flexible shielded cable connects the thermistor to circuitry which is responsive to thermally induced changes in its resistance. The cable shield is electrically connected at one end to the probe housing and at the other end to a wall of the oven cooking cavity. In the embodiments disclosed in the above-mentioned Chen and Fitzmayer U.S. Pat. No. 3,975,720, the total effective electrical length of the probe and the cable, measured along the cable and probe from the cooking cavity wall to the distal end of the probe, is selected to be approximately equal to n .lambda./2, where n is any integer and .lambda. is the wavelength of the microwaves being used to cook the food in the oven. Such a probe and cable assembly may successfully be operated in a microwave oven with minimal direct heating of the probe and cable by the microwave energy.
In general, probes of the above-described type operate satisfactorily and do minimize problems caused by direct heating of the probe and cable by the microwave energy. However, when they are used to monitor the internal temperature of certain foods, including meats and baked potatoes, overcooking of the food in the region immediately adjacent the probe sometimes occurs. The affected region generally extends along the length of the probe, but it is cone-shaped, being wider at the surface of the food. This is considered undesirable from the standpoint of appearance of the food after it has been cooked. Further, the temperature sensing probe necessarily responds to the temperature of the food in the region immediately adjacent the probe. When the food in this immediately adjacent region cooks faster than the other regions of the food, there is an indication of doneness before the bulk of the food (all the other regions within the food) is fully cooked to the desired degree of doneness. The condition which results in overcooking of the food in the region immediately adjacent the probe has been termed "feed in" because the cable and probe seemingly pick up microwave energy present within the cooking cavity and feed or direct the energy into the food along the length of the probe.
The above-described "feed in" effect is to be distinguished from a premature indication of doneness due to direct heating of the probe by the microwave energy, one of the problems to which the inventions comprising the subject matter of the above-mentioned U.S. Pat. Nos. 3,975,720 and 3,974,696 are directed. When a "feed in" problem occurs, the probe may be accurately sensing temperature; the temperature of the food which the probe is sensing (immediately surrounding the probe) is simply not the temperature of the food as a whole.
When the above-mentioned "feed in" problem is manifested, overcooking of the food occurs along the length of the probe in a cone-shaped region beginning at the surface where the probe is inserted into the food. The degree of overcooking is not easily predicted in any particular case and appears to be partially dependent upon the particular combination of size of food and particular insertion depth of the probe into the food. Voltage and current distributions are set up along the cable and probe due to the presence of the microwave energy field, and it is believed that these distributions vary somewhat unpredictably under different cooking conditions. In other words, unless conditions such as size and type of food and exact placement of the probe are maintained exactly the same from one cooking operation to the next, seemingly inconsistent results have been found to occur.
By the present invention, there is provided a temperature-sensing probe assembly for monitoring internal temperature of food while it is being cooked in a microwave oven, which probe assembly reliably avoids the above-described "feed in" problem, even when the probe is used to measure the internal temperature of many different types of food and under varying cooking conditions.